This invention relates generally to a process for preparing a mask used in the fabrication of semiconductor devices, and more particularly to a process for preparing a mask, such as a light-shielding mask for use as a photo-mask to be applied to the various photo-etching processes involved in the manufacture of a semiconductor device and a reaction-resistive mask for use as a protective mask for use in the anodic oxidation process and the porous silicon formation process in the manufacture of the semiconductor device.
Selective-exposure type photo-masks, which are widely used in a photo-etching process, include a dark portion, or a shielding portion, against ultraviolet rays and/or visible light, in a predetermined pattern. The known photo-masks include the emulsion type photo-mask using silver halogenide for the dark portion and the hard photo-mask using chromium metal, chromium oxide, cadmium sulfide, iron oxide, chalcogen glass and the like for the dark portion. However, the emulsion type photo-mask has a poor resolving power because of the large size of the silver particles and the large thickness (5 to 10 .mu.m) of the emulsion layer, and also, has a short life because the emulsion layer is susceptible to damage when used in close contact with a wafer to be masked. The hard photo-mask has an improved resolving power because it employs a relatively thin (1000 to 5000A) film of a metal or metal oxide for the dark portion, and has an increased mechanical strength and a life of 6 to 10 times greater than that of the emulsion type photo-mask. However, the hard photo-mask also does not have a sufficiently long useful life and its manufacturing cost is 8 to 10 times greater than that of the emulsion type photo-mask, so that the total cost of manufacture of a photo-mask for use in processing a semi-conductor substrate, in terms of the same number of wafers to be processed, is substantially the same as that of the emulsion type photo-mask. The hard photo-mask is particularly disadvantageous in that, in the case of a chromium photo-mask using a metal such as chromium for a dark portion, high optical-reflectance results. As a result, when the hard photo-mask is used for a close-contact exposure, the reflected light from a wafer is again reflected at the surface of the chromium causing multi-reflection, thereby impairing the reproducibility of the pattern to be transferred. In addition, with the other types of hard photo-masks described, the surface strengths of chalcogen glass and cadonium sulfide are inferior to that of the chromium photo-mask. On the other hand, the resistance of iron oxide against chemical is low in the event of an attack of a hot liquid of a photo-resist removing agent (for instance, J-100 made by Indust-Re Chem. Labo. Co., or OMR removing agent made by Tokyo Oka Kogyo Co., Tokyo, Japan, while chromium oxide is attacked by a mask cleaning liquid such as a hot concentrated sulfuric acid, hot nitric acid and the like. In addition, when cleaning is repeatedly applied to a chromium oxide photo-mask for repeated use, the optical density is lowered, thereby seriously impairing the function as a photo-mask. In addition, since either an evaporation process, a reactive sputtering process, or a chemical vapor deposition process is used in the manufacture of the hard photo-mask, strict control is required for the thickness of a film as well as for optical density.
In a widely used process for preparing an electrode wiring for a semi-conductor, particularly for an integrated circuit, a metal layer such as aluminum is subjected to selective etching, with a photo-resist film used as a protective mask. An example of the use of the photo-mask is the selective photo-etching of this aluminum layer in which the photo-mask is used to selectively expose the photo-resist film applied over the aluminum layer.
A process for preparing an electrode wiring according to selective anodic oxidation in place of the selective photo-etching has recently been proposed. In this process, an insulating film is selectively formed on a layer of an anodizable material such as aluminum, tantalum, or silicon and then the anodizable material layer is selectively converted into an oxide by anodic oxidation with the insulating film used as a protective mask against oxidation. Since a photo-resist film is used as the insulating protective film, the electrolyte tends to make ingress during the anodic oxidation into the side lower portion of the photo-resist film, because of poor adhesion between the photo-resist film and the anodizable material layer, with the result that an oxidized layer is formed even at a portion which is covered with the photo-resist film, thus resulting in a failure to prepare a desired electrode wiring pattern. In other words, the mere use of a photo-resist film will not insure satisfactory anodic oxidation for a long period of time, thus presenting a vital shortcoming for the anodic oxidation.
A selective preparation of a porous silicon layer, in which a silicon substrate is selectively converted into a porous layer according to the anodic reaction in a high concentration of hydrofluoric acid or like solution, has recently been proposed as a technique in manufacture of devices. In this selective conversion process, the silicon substrate is selectively covered with an insulating layer and then is subjected to an anodic reaction treatment with the insulating film used as a protective mask against the anodic reaction. As a material for the insulating protective film which must resist against a high concentration of hydrofluoric acid, a photo-resist, silicon oxide, silicon nitride, or alumina may be considered. However, materials other than alumina undergo a change in composition of a film or etching during the anodic reaction in hydrofluoric acid solution, so that those materials fail or retain their functions as protective masks for a long period of time. Moreover, an alumina film presents a poor contact with silicon, resulting in peeling of the film during the anodic reaction. Therefore, the alumina film is also unsatisfactory for use as a protective mask.